How bad can a relationship between two military allies get? If this year's tally of incidents is anything to go by, Pakistan's rage against the American military machine can get a lot worse. First came the affair over Raymond Davis, the CIA agent who shot dead two men who had pulled up in front of his car at a traffic light in Lahore. Then came the US raid that killed Osama bin Laden in the Pakistani garrison town of Abbottabad. And now this.

An Afghan special forces operation, backed up by Nato troops, allegedly came under fire from across the border. Afghan troops, another report goes, called in Nato airstrikes, and two Pakistani military posts were hit, killing 24 soldiers. The reaction in Pakistan ranged yesterday from cold fury (it is just not believed in Pakistani military circles that Nato was unaware of the co-ordinates of the two military posts in the village of Salala) to hot conspiracy: America was the "big evil". The politician Imran Khan told thousands of supporters on Saturday that it was time to end the alliance with the US. It would be folly to dismiss this as mere populism. After a year like this, the Pakistani military will have to cope with rising levels of pressure from within its own ranks to end co-operation with the US.

The Afghan element to this tale of friendly fire is also troubling. If, as US forces start to draw down, Afghan troops take the lead in highly sensitive areas like these, where the exact line of the border is unclear, then this weekend's woeful events may not be the last. As it is, it would not take much for Pakistani and Afghan troops to open fire on each other. On the Afghan side of the border in Kunar province, there was little doubt that the US military had done the right thing. They were congratulated for hitting the right target. On Saturday the Afghan president, Hamid Karzai, approved a second list of areas where Afghan forces will start taking the lead from Nato troops. As it is, the US is reluctant to give the Afghans fast jets or heavy artillery for fear of what they would do.

The short-term response is not as troubling as the long-term implications. Pakistan closed two border crossings and gave the US 15 days to quit Shamsi airbase in Baluchistan, from which it flew drones targeting militants in the tribal areas. The closures will make Barack Obama more dependent on Vladimir Putin's goodwill, and the northern supply route through which 60% of troops and military cargo to Afghanistan now travel. But, of itself, the closures will be a temporary problem. Of greater significance is the erosion of Pakistani public support for the US fight against the Taliban. It would not be the first strategic mistake the US had made in this war, but it could yet prove the costliest.